Remove confusing A2DP check for fast mixer

From a dumpsys of audioflinger, it seems that bluetooth a2dp is using
fast mixer. However, from inspecting the code, it seems like this is
not the case. After adding log statements, I noticed that outDeviceTypes()
always returns nothing in this block. It seems that the statement is
not fully ready by the time that this block is called. Later on,
outDeviceTypes() seems to work fine but not here.

The BT audio HAL should no longer be bursty because of two changes
1. In the Qualcomm and newer HALs, the offload path has been added so
the DSP can encode the data instead. This path is not bursty.
2. When offloading is not implemented, the Bluetooth a2dp hal is used.
In system/bt, audio_a2dp_hw has been replaced by audio_bluetooth_hw
and this HAL is not bursty.

This means that using FastMixer for BT is good and the only "fix" needed
here is to remove this confusing code. I will do a sweep for other
instances of outDeviceTypes() in a future cl.

Bug: 195790812
Test: Tested on Bramble with various headsets and offloading turned
on/off

Change-Id: I773028ba3b79fa6f7fc4244177b24751d373d38d